1. Field of the Invention
The present invention relates in general to an object having a refractive index which varies radially from a central portion of the object toward a peripheral portion thereof, and a method and apparatus for producing the same. More particularly, the present invention relates to a plastic optical fiber preform having a refractive index which varies radially from the central portion of the object toward a peripheral portion thereof, and to a method and apparatus for producing the preform.
2. Description of the Prior Art
Optical fibers that are used as an optical transmission line are classified into single-mode fibers and multi-mode fibers depending on their optical signal transmission mode. Optical fibers that are currently used for long-range optical communication are mainly step-index single optical fibers based on quartz glass. These quartz glass optical fibers are fine fibers having a diameter of only about 5 to 10 xcexcm. for this reason, it is very difficult and expensive to achieve an alignment and connection for those fibers.
On the other hand, multi-mode glass optical fibers having a larger diameter than that of the single-mode glass optical fibers may be used for short-range communication such as LAN (local area network), but they have a limited application because they have shortcomings in that they also have high costs for their connection and are fragile. Thus, for the short-range communication within a range of 200 M, for example LAN, metal cables such as twisted pair or coaxial cables are mainly used. However, such metal cables have an information transmission speed (or bandwidth) of about 150 Mbps at most, so that they cannot meet future standards of, for example, 625 Mbps that is a standard of ATM (Asynchronous Transfer Mode) in the 2,000""s.
For these reasons, in the United States of America and Japan, etc., there have been made many efforts and investments over a past decade to develop plastic optical fibers which can be used for short-range communication such as LAN. By virtue of a flexibility of the polymer material, the plastic optical fibers can be reached the range of 0.5 mm to 1.0 mm in diameter, which is greater than those of glass optical fibers by 100 times or more. Thus, it is easy to achieve an alignment and connection for these plastic optical fibers. For such plastic optical fibers, polymer connectors manufactured by extrusion-moulding may also be used. Thus, a significant cost saving is expected. The plastic optical fibers can have either a step-index (SI) type having a refractive index profile in which the refractive index is stepwise radially changed, or a graded-index (GI) type having a refractive index profile in which the refractive index is gradually radially changed. The SI plastic optical fiber is high in its modal dispersity and thus cannot faster in its signal transmission speed (or bandwidth) than that of the cables, whereas the GI plastic optical fiber is low in its modal dispersity so that they can have a high transmission speed. Accordingly, the GI plastic optical fiber is known to be suitable for a short-range, high-speed communication medium by virtue of its large diameter and thus a cost-saving effect, and the low modal dispersity.
Prior processes for the production of the GI plastic optical fibers include an interfacial gel polymerization process which was first reported by Koike, a professor of Geio University in Japan, and several patent applications associated with the interfacial gel polymerization process (see, Koike, Y. et al., Applied Optics, Vol. 27, 486 (1988)). The interfacial gel polymerization process comprises providing a matrix polymer and a relatively low molecular weight dopant which is non-polymerizable with the matrix and has a refractive index different from the matrix, distributing the dopant in the matrix in a radial direction to make a preform with a radially varying refractive index, and heating and drawing the preform into fiber.
Meanwhile, Japanese Patent Application Laid Open Heisei 04-86603 discloses a method of producing a GI plastic optical fiber, comprising preparing a polymer fiber by an extrusion process, and then extracting a low molecular material contained in the polymer fiber in a radial direction or introducing a low molecular weight material into the polymer fiber in a radial direction.
Moreover, in 14th Annual Meeting of the Polymer Processing Society, Yokohama, Japan (June, 1998), Park and Walker have reported that a GI plastic optical fiber could be produced by a continuous process in which a refractive index profile is created by a mechanical mixing using a specific coextrusion die called xe2x80x9ca GRIN die blockxe2x80x9d.
In addition, WO 97/29903 of which inventors are Van Duijnhoven and Bastiaansen discloses a method of polymerizing monomers of a different density and refractive index under a centrifugal field, to create a density gradient, and thus a concentration gradient, thereby creating a refractive index gradient.
In order to maximize a bandwidth of a GI plastic optical fiber, the GI plastic optical fiber must have a radial refractive index profile close to a parabola as shown in FIGS. 1a and 1b. The refractive index profile is theoretically determined according to a power-law index model represented by the following equation (1) (see, Halley, P., Fiber Optics System, J. Wiley and Sons (1987)):                                                                         n                ⁡                                  (                  r                  )                                            =                                                                    n                    1                                    ⁡                                      [                                          1                      -                                              2                        ⁢                                                                              Δ                            ⁡                                                          (                                                              r                                a                                                            )                                                                                g                                                                                      ]                                                                    1                  2                                                                                        r              ≤              a                                                                                          n                ⁡                                  (                  r                  )                                            =                              n                2                                                                        r               greater than               a                                                          (        1        )            
where r represents a distance from a center of a cylindrical fiber, a represents a radius of the fiber, n1 and n2 represent indexes at r=0 and r=a, respectively, and n1 is greater than n2. 2xcex94=(n12xe2x88x92n22)/n12, and g is a power-law index. According to the g value, the radial refractive index profile is determined. When the g value is 2, the power-law is named xe2x80x9cparabolic lawxe2x80x9d. At the g value approaching 2, an optimal refractive index profile can be reached at which the bandwidth is maximized. In is this case, when an optical signal as a delta function is input into the GI plastic optical fiber, the maximum bandwidth is given by the following equation (2):                     B        =                              c                          0.88              ⁢              Lnl                                ⁢                      1                          Δ              2                                                          (        2        )            
wherein L is a length of the optical fiber, c is a velocity of light, n is a refractive index, and l is a Debye correlation length of a polymer used.
The bandwidth of the GI plastic optical fiber as theoretically described above is sensitively changed with the g value of the power law index model. Thus, in a process of producing the GI plastic optical fiber, the ability to control the g value, i.e., the ability to control a radial refractive index profile, is important to obtain a greater bandwidth of the prepared optical fiber. In all the existing GI preform producing processes other than the process by Park and Walker as described above, the radial refractive index profile is determined according to a diffusivity of a low molecular material or a relative chemical reactivity between the low molecular weight material and the high molecular weight material, and thus the process itself does not have the ability to control the g value (i.e., the ability to control the radial refractive index profile). On the other hand, the producing process by Park and Walker that is a mechanical mixing method is known to have the ability to control the g value by itself. However, this process results in a contamination of the optical fiber with foreign materials due to the pyrolysis of the polymer which is caused by complex structure of a specific extrusion die called xe2x80x9cGRIN die blockxe2x80x9d and by a continuous coextrusion process. Thus, it is difficult for such processes to prepare the optical fiber having a low attenuation of an optical signal.
It is therefore an object of the present invention to provide to solve the above problems with the prior art and to provide a method of producing an object having a refractive index which varies radially from a central portion of the object to a peripheral portion thereof, thereby providing a new method of producing an GI plastic optical fiber which enables an easy control of a radial refractive index profile of the plastic optical fiber.
In one aspect of the present invention, there is provided a method of producing an object having a refractive index which varies radially from a central portion of the object toward a peripheral portion thereof, the method comprising the steps of: mounting in a rotatable reactor a solid central rotating body formed by polymerizing a first component; filling a liquid second component in the rotatable reactor around the central rotating body; rotating the central rotating body and/or the rotatable reactor to subject the first component to a dissolution, diffusion, and radial mixing into the second component; and polymerizing the dissolved first component and the second component.
In another aspect of the present invention, there is provided objects with a radially varying refractive index that are prepared by the method according to the first aspect.
In other aspect of the present invention, there is provided an apparatus of producing an object having a refractive index which varies radially from the central portion of the object toward the peripheral portion thereof, the apparatus comprising: a rotatable reactor mounted with a solid central rotating body formed by polymerizing a first component and filled with a liquid second component around the central rotating body, and the central rotating body being dissolved by a rotation of the central rotating body and/or the rotatable reactor to be mixed with the second component, and the resultant mixture of the dissolved central rotating body and the second component being polymerized to produce the object; means for driving the rotatable reactor; means for driving the central rotating body, and means for fixing the central rotating body to the driving means therefor.